Wavelength conversion member and photovoltaic device using same

ABSTRACT

A wavelength conversion member ( 20 ) includes a fluoride phosphor ( 25 ) activated by Ce 3+  or Eu 2+ . Then, with regard to the fluoride phosphor, internal quantum efficiency measured at 80° C. is 85% or more when internal quantum efficiency measured at 30° C. is taken as 100%. Moreover, a photovoltaic device includes the above-mentioned wavelength conversion member. The wavelength conversion member uses a fluoride phosphor, in which a decrease of the internal quantum efficiency is suppressed, and excellent temperature characteristics are imparted. Therefore, the wavelength conversion member can effectively utilize ultraviolet light even at high temperature, and it becomes possible to enhance an output of the photovoltaic device.

TECHNICAL FIELD

The present invention relates to a wavelength conversion member and aphotovoltaic device using the same. More specifically, the presentinvention relates to a wavelength conversion member that enhancesphotoelectric conversion efficiency thereof, and to a photovoltaicdevice using the wavelength conversion member.

BACKGROUND ART

In general, in a solar cell, photoelectric conversion efficiency ofultraviolet light is lower than photoelectric conversion efficiency ofvisible light. For example, in the solar cell, in the ultraviolet lightin which a wavelength ranges from 300 nm or more to less than 400 nm,the photoelectric conversion efficiency is low, and in the region of thevisible light and the infrared light, in which a wavelength ranges from400 nm or more to less than 1200 nm, the photoelectric conversionefficiency is high. Moreover, such ultraviolet light in which awavelength stays within a range less than 380 nm is prone to damage thesolar cell. Accordingly, in the conventional solar cell, the ultravioletlight in which the wavelength stays within the range less than 380 nmhas been cut, for example, by means of a filter.

However, if the ultraviolet light in which the wavelength stays withinthe range less than 380 nm can be used for power generation, then thephotoelectric conversion efficiency of the solar cell is expected to beimproved. Therefore, in recent years, in the solar cell, it has beenstudied not just to cut the ultraviolet light in which the wavelengthstays within the range less than 380 nm, but to convert the ultravioletlight into long-wavelength light and to use the long-wavelength lightfor power generation. Specifically, there has been studied a technologyfor providing a wavelength conversion layer that converts theultraviolet light into the visible light or the infrared light.

As a phosphor, which is used for the wavelength conversion layer asdescribed above, and converts the ultraviolet light into the visiblelight or the infrared light, there is disclosed barium halide in whichEu²⁺ is activated, and specifically, barium fluoride in which Eu²⁺ isactivated (for example, refer to Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. H02-503717

SUMMARY OF INVENTION

However, the barium fluoride in which Eu²⁺ is activated exhibits lightemission only at low temperature, and hardly exhibits light emission ata temperature of 25° C. or more, and accordingly, has not been able toconvert the ultraviolet light into the visible light or the infraredlight sufficiently. That is, it has been difficult for the wavelengthconversion member using the conventional fluoride phosphor to enhance anoutput of the solar cell at high temperature.

The present invention has been made in consideration of such problems asdescribed above, which are inherent in the prior art. Then, it is anobject of the present invention to provide a wavelength conversionmember capable of increasing the photoelectric conversion efficiencyeven at high temperature, and enhancing the output of the solar cell,and to provide a photovoltaic device using the wavelength conversionmember.

In order to solve the above-described problems, a wavelength conversionmember according to a first aspect of the present invention includes afluoride phosphor activated by Ce³⁺ or Eu²⁺. Then, with regard to thefluoride phosphor, when internal quantum efficiency measured at 30° C.is taken as 100%, internal quantum efficiency measured at 80° C. is 85%or more.

A photovoltaic device according to a second aspect of the presentinvention includes the above-mentioned wavelength conversion member.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an Example of asolar cell module as a photovoltaic device according to an embodiment ofthe present invention.

FIG. 2 is a graph showing relationships between relative internalquantum efficiency and temperature in phosphors of Example 3 andComparative Example 1.

FIG. 3 is a graph showing an emission spectrum and an excitationspectrum in the phosphor of Example 3.

DESCRIPTION OF EMBODIMENTS

A description will be made below in detail of a wavelength conversionmember and a photovoltaic device according to this embodiment. Note thatdimensional ratios in the drawings are exaggerated for convenience ofexplanation, and are sometimes different from actual ratios.

[Wavelength Conversion Member]

The wavelength conversion member according to this embodiment absorbsultraviolet light in sunlight, and thereafter, converts the ultravioletlight into visible light or infrared light. In this way, the visiblelight or the infrared light, in which spectral sensitivity is high,increases, and accordingly, it becomes possible to enhance photoelectricconversion efficiency of the solar cell. However, it is necessary forthe wavelength conversion member to exhibit high transmittance for thevisible light and the infrared light, which are a wavelength regionwhere the solar cell has high sensitivity. This is because, if thetransmittance of the solar cell decreases when the wavelength conversionmember is provided, then the photoelectric conversion efficiency islowered due to the decrease of the transmittance rather than enhancementof the photoelectric conversion efficiency by the wavelength conversionmember.

Therefore, in order to strike a balance between enhancement ofabsorption efficiency of the ultraviolet light by the wavelengthconversion member and suppression of the decrease in the transmittanceof the visible light and the infrared light, the decrease being causedthereby, the wavelength conversion member according to this embodimentincludes a fluoride phosphor activated by cerium ions (Ce³⁺) or europiumions (Eu²⁺). The fluoride phosphor is used as the wavelength conversionmember, whereby the decrease in the transmittance of the visible lightand the infrared light can be suppressed.

Moreover, in the wavelength conversion member in this embodiment, in thefluoride phosphor, when internal quantum efficiency measured at 30° C.is taken as 100%, internal quantum efficiency measured at 80° C. is 85%or more. In this embodiment, a fluoride phosphor having excellenttemperature characteristics is used, and accordingly, it becomespossible to sufficiently absorb the ultraviolet light even at hightemperature in the summer season, to convert the wavelength thereof, andto enhance the output of the solar cell. Note that, in the fluoridephosphor, when the internal quantum efficiency measured at 30° C. istaken as 100%, the internal quantum efficiency measured at 80° C. ispreferably 90% or more, more preferably 95% or more. The fluoridephosphor has so high internal quantum efficiency as described above,whereby such a wavelength conversion member can be obtained, whichfurther enhances the output of the solar cell at high temperature.

Here, it is necessary for the fluoride phosphor to use cerium ions(Ce³⁺) or europium ions (Eu²⁺) as an emission center. Ce³⁺ and Eu²⁺ takethe mechanism of light absorption and emission, which are based on4f_(n)

4f_(n-1)5d₁ allowed transition. Therefore, wavelengths of the absorptionand the light emission are changed depending on host crystals in whichthese are activated. Hence, Ce³⁺ or Eu²⁺ is used as the emission center,and appropriate host crystals are selected, whereby it becomes possibleto convert light in a region from near ultraviolet to violet into lightin such a wavelength sensed with high sensitivity by the solar cell.Note that, in the above-mentioned 4f_(n)

4f_(n-1)5d₁ allowed transition, Ce³⁺ corresponds to n=1, and Eu²⁺corresponds to n=7.

It is preferable that a host of the fluoride phosphor in this embodimentbe a fluoride containing alkaline earth metal and magnesium. That is, itis preferable that the host of the fluoride phosphor be a fluoridecontaining at least one element selected from the group consisting ofcalcium (Ca), strontium (Sr) and barium (Ba), and magnesium. This ispreferable since high light emission intensity is obtained even at hightemperature, and in addition, the quantum efficiency is increasedeasily. Note that a reason why the light emission intensity at hightemperature becomes good is presumed to be as follows. A crystalstructure of the fluoride phosphor becomes a strong host crystal inwhich a large number of [MgF₆]⁴⁻ units with an octahedral structure arebonded to one another while sharing F. Then, a phosphor, which iscomposed in such a manner that Ce³⁺ or Eu²⁺ as the emission center issubstituted for a part of this strong host crystal, has a strong crystalstructure, and accordingly, Ce³⁺ or Eu²⁺ becomes hard to vibrate. WhenCe³⁺ or Eu²⁺ as the emission center is hard to vibrate, the lightemission is stably performed even if the temperature of the phosphorrises. Accordingly, it is presumed that the phosphor composed of thefluoride containing such an alkaline earth metal element and magnesiumhas good light emission intensity.

Here, as the fluoride phosphor that uses Eu²⁺ as the emission center,CaF₂:Eu²⁺ has been heretofore known. In the case of this phosphor, sucha host compound is calcium fluoride (CaF₂), and the emission center isEu²⁺. Then, CaF₂: Eu²⁺ absorbs ultraviolet light having a wavelength of300 nm or more to less than 400 nm and emits visible light ofapproximately 425 nm, and accordingly, can be used as a wavelengthconversion material for the solar cell. Moreover, a refractive index ofCaF₂ is 1.43, which is approximate to a refractive index of a sealingmaterial to be described later, and accordingly, a wavelength conversionmember is obtained, which hardly reduces the transmittance of thevisible light and the infrared light even if CaF₂: Eu²⁺ is dispersed inthe sealing material.

However, by means of the wavelength conversion member using CaF₂: Eu²⁺,the output of the solar cell cannot be enhanced sufficiently. A reasonwhy the output of the solar cell cannot be enhanced by means of theconventional fluoride phosphor as described above is temperaturequenching of the phosphor. The temperature quenching is a phenomenon inwhich the internal quantum efficiency decreases as the temperature ofthe phosphor rises. The temperature of the solar cell may rise to 80° C.or more depending on a usage environment. Therefore, when thetemperature quenching of the phosphor for use in the wavelengthconversion member is conspicuous, then the enhancement of the efficiencyby the wavelength conversion cannot be sufficiently obtained under theusage environment where the solar cell gets hot. For example, theinternal quantum efficiency of CaF₂: Eu²⁺ at 80° C. decreases to 80% orless of the internal quantum efficiency thereof at 30° C., andaccordingly, the output of the solar cell has not been able to beenhanced sufficiently. Moreover, other conventional fluoride phosphorsare conspicuous in temperature quenching, and cannot exhibit at highinternal quantum efficiency at 80° C. or more. Therefore, a wavelengthconversion member, which is capable of sufficiently enhancing the outputof the solar cell, has not been able to be obtained.

In contrast, in the wavelength conversion member in this embodiment, thefluoride phosphor is activated by Ce³⁺ or Eu²⁺, and when the internalquantum efficiency measured at 30° C. is taken as 100%, the internalquantum efficiency measured at 80° C. is 85% or more. Therefore, thewavelength conversion is performed efficiently even under the usageenvironment where the solar cell gets hot, and it becomes possible toenhance the output of the solar cell. Note that, as far as the inventorof the present invention knows, there is no reported Example of such afluoride phosphor activated by Ce³⁺ or Eu²⁺, which has small temperaturequenching, and it has never been thought that there is a phosphor thatstrikes a balance between a low refractive index and good temperaturequenching characteristics.

Here, in general inorganic phosphors, those using rare earth ions otherthan Ce³⁺ and Eu²⁺ as the emission center are also known. However, ifthe emission center is the rare earth ions other than Ce³⁺ and Eu²⁺,even if a composition of the host crystal is adjusted, it is difficultto obtain an inorganic phosphor that absorbs the ultraviolet lighthaving a wavelength of 300 to 400 nm. However, when at least one of Ce³⁺and Eu²⁺ is contained, the composition of the host crystal is adjusted,whereby it becomes possible to obtain the inorganic phosphor thatabsorbs the ultraviolet light having a wavelength of 300 to 400 nm. Amain reason for this is presumed as follows.

Among the rare earth ions, rare earth ions of Ce to Yb have electrons ina 4f orbital. The absorption and emission of light, which are derivedfrom the rare earth ions, are classified into two types, which are:transition in a 4f shell; and transition between a 5d shell and the 4fshell.

The ions other than Ce³⁺ and Eu²⁺ among the rare earth ions generallyabsorb and emit light by the transition in the 4f shell. However, in thetransition in the 4f shell, the electrons in the 4f orbital are presentin an inside of electrons in a 5s orbital and a 5p orbital, and areshielded, and accordingly, fluctuation of an energy level due to asurrounding influence are less likely to occur. Therefore, in theinorganic phosphor that uses the ions other than Ce³⁺ and Eu²⁺ as theemission center, even if the composition of the host crystal isadjusted, the change of the light emission wavelength is small, and itis difficult to obtain the inorganic phosphor that absorbs theultraviolet light having a wavelength of 300 to 400 nm.

In contrast, Ce³⁺ and Eu²⁺ perform the absorption and emission of lightby the transition between the 5d shell and the 4f shell, that is,transition between 4f^(n) and 4f^(n-1)5d. In this transition between the5d shell and the 4f shell, the 5d orbital is not shielded from otherorbits, and accordingly, the fluctuation of the energy level of the 5dorbital due to the surrounding influence is likely to occur. Therefore,in the inorganic phosphor that uses Ce³⁺ and Eu²⁺ as the emissioncenter, in the case of light emission that is based on the transitionfrom the 4f^(n-1)5d¹ level to the 4f orbital, it becomes possible togreatly change the light emission wavelength by adjusting thecomposition of the host crystal. Due to this great change of the lightemission wavelength, in accordance with the inorganic phosphor that usesCe³⁺ and Eu²⁺ as the emission center, it becomes possible to obtain theinorganic phosphor, which absorbs the ultraviolet light having awavelength of 300 to 400 nm.

In the wavelength conversion member of this embodiment, it is preferablethat the fluoride phosphor include a compound, which is represented byGeneral formula (1), as the host. In this way, a fluoride phosphor canbe obtained, which is excellent in absorption rate of the ultravioletlight, quantum efficiency and temperature characteristics. Note that, inGeneral formula (1), it is preferable that the alkaline earth metal beat least one element selected from the group consisting of calcium (Ca),strontium (Sr) and barium (Ba):

M₃Mg₄F₁₄  (1)

(where M is alkaline earth metal).

Note that a crystal structure, in which one of Ce³⁺ and Eu²⁺ issubstituted for a part of atoms of the host crystal having thecomposition represented by General formula (1), is represented, forexample, by Chemical formula (2) or (3):

(M_(1-x)Ce_(x))₃Mg₄F₁₄  (2)

(where M is at least one alkaline earth metal selected from the groupconsisting of Ca, Sr and Ba, and x satisfies 0<x<0.3.).

(M_(1-y)Eu_(y))₃Mg₄F₁₄  (3)

(where M is at least one alkaline earth metal selected from the groupconsisting of Ca, Sr and Ba, and y satisfies 0<y<0.3.).

In Chemical formula (2), x is preferably 0.003<x<0.1, more preferably0.01<x<0.1. Moreover, in Chemical formula (3), y is preferably0.003<y<0.1, more preferably 0.01<y<0.1.

Moreover, the fluoride phosphor may contain alkaline metal. This makesit possible to control an excitation spectrum and an emission spectrum,which are derived from Eu²⁺ and Ce³⁺. Note that, preferably, thealkaline metal is at least one element selected from the groupconsisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb)and cesium (Cs).

The fluoride phosphor may contain a halogen element other than fluorinewithin a range where the crystal structure of the fluoride phosphor isnot damaged. This makes it possible to control the excitation spectrumand the emission spectrum, which are derived from Eu²⁺ and Ce³⁺, and inaddition, to control the refractive index of the phosphor. Note that,preferably, the halogen element is at least one element selected fromthe group consisting of chlorine (Cl), bromine (Br) and iodine (I).

The fluoride phosphor may contain manganese ions (Mn²⁺). In this way,energy is transferred from Eu²⁺ or Ce³⁺ to Mn²⁺, and it becomes possiblefor Mn²⁺ to serve as the emission center and to emit light on the longwavelength side. Moreover, the fluoride phosphor may contain oxygenwithin the range where the crystal structure of the fluoride phosphor isnot damaged. This makes it possible to control the refractive index ofthe phosphor.

Moreover, the fluoride phosphor may contain a rare earth element otherthan the element serving as the emission center. The fluoride phosphorcontains the rare earth element, whereby a large amount of the elementserving as the emission center can be contained, and it becomes possibleto enhance the absorption rate of the ultraviolet light. Note that,preferably, the rare earth element is at least one element selected fromthe group consisting of scandium (Sc), yttrium (Y), lanthanum (La),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

The fluoride phosphor may contain an element capable of taking ahexacoordinated state, the element being other than Mg. The fluoridephosphor contains such an element as described above, whereby it becomespossible to control the refractive index of the phosphor. Note that,preferably, the element capable of taking the hexacoordinated state is,for example, at least one element selected from the group consisting ofaluminum (Al), gallium (Ga), scandium (Sc), zirconium (Zr), manganese(Mn) and lutetium (Lu).

It is preferable that the fluoride phosphor have the same crystalstructure as Pb₃Nb₄O₁₂F₂. In this way, the fluoride phosphor can beobtained, which is excellent in absorption rate, quantum efficiency andtemperature characteristics.

Moreover, it is preferable that the fluoride phosphor include acompound, which is represented by Chemical formula (4), as the host. Inthis way, the fluoride phosphor can be obtained, which is excellent inabsorption rate, quantum efficiency and temperature characteristics.Moreover, it becomes possible to adjust the refractive index of thefluoride phosphor:

Ba₂(Ca_(1-x)Sr_(x))Mg₄F₁₄  (4)

(where 0<x<1.).

It is also preferable that the fluoride phosphor include a compound,which is represented by Chemical formula (5), as the host. This makes itpossible to adjust the refractive index of the fluoride phosphor:

Ba_(2+y)(Ca_(1-x)Sr_(x))_(1-y)Mg₄F₁₄  (5)

(where 0<x<1, 0<y<1).

In the wavelength conversion member of this embodiment, a centralparticle size (D₅₀) of the fluoride phosphor is preferably 0.1 um ormore to less than 100 um, and more preferably 0.3 um or more to lessthan 30 um. The central particle size of the phosphor stays within thisrange, whereby it becomes possible to obtain such a wavelengthconversion member that sufficiently absorbs the ultraviolet light in thesunlight and suppresses the decrease of the transmittance of the visiblelight and the infrared light. Note that the central particle size of thefluoride phosphor can be measured, for example, by a laserdiffraction/scattering-type particle size distribution measurementapparatus.

Moreover, an average particle size of the fluoride phosphor ispreferably 0.1 um or more to less than 100 um, and more preferably 0.3um or more to less than 30 um. The average particle size of the fluoridephosphor stays within this range, whereby it becomes possible to obtainsuch a wavelength conversion member that sufficiently absorbs theultraviolet light in the sunlight and suppresses the decrease of thetransmittance of the visible light and the infrared light. Note that theaverage particle size of the fluoride phosphor is defined as an averagevalue of longest axis lengths in arbitrary 20 or more phosphor particlesobserved by a scanning electron microscope.

It is preferable that the light emission wavelength of the fluoridephosphor be 440 nm or more. In this way, the ultraviolet light can beconverted into a region where the spectral sensitivity of the solar cellis high, and accordingly, it becomes possible to greatly enhance theoutput of the solar cell.

The refractive index of the fluoride phosphor is preferably 1.41 or moreto less than 1.57, more preferably 1.44 or more to less than 1.54, andparticularly preferably 1.47 or more to less than 1.51. In this way,when the fluoride phosphor is dispersed in the sealing material asdescribed later, it becomes possible to suppress the decrease of thetransmittance of the visible light and the infrared light.

It is preferable that the wavelength conversion member of thisembodiment further include the sealing material that disperses thefluoride phosphor therein. That is, in the wavelength conversion member,it is preferable that the fluoride phosphor be dispersed in the sealingmaterial. The fluoride phosphor is dispersed in the sealing material,whereby it becomes possible to sufficiently absorb the ultravioletlight, and to perform the wavelength conversion for the visible light orthe infrared light. Furthermore, it becomes easy to form the wavelengthconversion member into a sheet shape or a film shape, and it becomespossible to dispose the wavelength conversion member on the solar cellwith ease.

As the sealing material, for example, there can be used at least oneresin material selected from the group consisting of an ethylene-vinylacetate copolymer (EVA), polyvinyl butyral (PVB), polyimide,polyethylene, polypropylene, and polyethylene terephthalate (PET). Notethat refractive indices of these resins are 1.41 or more to less than1.57.

Here, when the wavelength conversion member has a configuration in whichthe fluoride phosphor is dispersed in the sealing material, then inorder not to lower the transmittance of the visible light and theinfrared light, it is preferable that such a particle size of thephosphor be reduced to approximately several tens of nanometers, or thatthe refractive index of the phosphor be approximately the same as thatof the sealing material. However, as the particle size of the phosphoris larger, a defect density in the phosphor is reduced, and an energyloss during the light emission is reduced, and accordingly, lightemission efficiency is increased. Hence, in order not to lower thetransmittance of the visible light and the infrared light, it ispreferable that, in the wavelength conversion member of this embodiment,the refractive index of the phosphor be approximately the same as thatof the sealing material. Specifically, as described above, therefractive index of the fluoride phosphor is preferably 1.41 or more toless than 1.57.

Herein, the inorganic phosphor is used for a variety of light emittingdevices, and is used for, for example, a fluorescent lamp, an electrontube, a plasma display panel (PDP), a white LED, and the like. Ingeneral, the inorganic phosphor is a compound in which an element thatcan become fluorescence-emitting ions is substituted for a part of acrystalline compound. As mentioned above, the ions having suchcharacteristics are called “emission center”. Then, the ions serving asthe emission center are introduced into the host serving as thecrystalline compound. As the host of the phosphor for use in such anapplication, there are mentioned oxides, nitrides, sulfides,oxynitrides, oxysulfides, acid halides, and the like. In general, arefractive index of each of these compounds serving as the hosts is 1.6or more, which is higher than that of the sealing material. Therefore,in the wavelength conversion member in which the phosphor composed ofthe host as described above is dispersed in the sealing material, thevisible light and the infrared light are reflected due to a differencein refractive index between the sealing material and the phosphor, andthis reflection has caused a deterioration of the photoelectricconversion efficiency due to the decrease of the transmittance.

In contrast, the wavelength conversion member of this embodiment employsthe fluoride phosphor as the phosphor. In the fluoride phosphor, afluoride having a refractive index as low as that of the sealingmaterial is the host compound. Therefore, since the difference inrefractive index between the sealing material and the phosphor is small,the reflection of the visible light and the infrared light is reduced inthe wavelength conversion member, and it becomes possible to suppressthe deterioration of the photoelectric conversion efficiency, which iscaused by the decrease of the transmittance.

In the wavelength conversion member of this embodiment, a content of thefluoride phosphor in the sealing material is preferably 0.1% by volumeor more to less than 10% by volume, more preferably 1% by volume or moreto less than 5% by volume. This makes it possible to obtain such awavelength conversion member that sufficiently absorbs the ultravioletlight and suppresses the decrease of the transmittance of the visiblelight and the infrared light.

The fluoride phosphor in the wavelength conversion member of thisembodiment can be produced by a publicly known method. Specifically,like yttrium aluminum garnet (YAG), the fluoride phosphor can besynthesized by using a publicly known solid phase reaction.

Specifically, first, a fluoride of an alkaline earth metal element, afluoride of a rare earth element, and a fluoride of magnesium areprepared. Next, raw material powders are blended so as to have astoichiometric composition of a desired compound or a composition closeto the stoichiometric composition, and are mixed thoroughly by using amortar, a ball mill or the like. Thereafter, such a mixed raw materialis baked by an electric furnace or the like while using a baking vesselsuch as an alumina crucible, whereby the fluoride phosphor of thisembodiment can be prepared. Note that, when the mixed raw material isbaked, it is preferable to heat the mixed raw material at a bakingtemperature of 700 to 1000° C. for several hours in the atmosphere or aweakly reductive atmosphere. Moreover, an additive such as a reactionaccelerator may be added to the raw materials. For example, ammoniumfluoride (NH₄F) is preferable since the ammonium fluoride suppressesdesorption of fluorine.

Then, the wavelength conversion member of this embodiment can beobtained by mixing the phosphor, which is obtained as described above,with the sealing material, and by molding an obtained mixture into asheet form, a film form, a plate form, or the like. Note that, though athickness of the wavelength conversion member is not particularlylimited, it is preferable to set the thickness, for example, to 200 μmto 1000 μm.

As described above, the wavelength conversion member according to thisembodiment includes the fluoride phosphor activated by Ce³⁺ or Eu²⁺.Then, in the fluoride phosphor, when the internal quantum efficiencymeasured at 30° C. is taken as 100%, the internal quantum efficiencymeasured at 80° C. is 85% or more. Then, it has been found out that somefluoride phosphors are excellent in temperature quenching as a result ofexamining in detail such Ce³⁺ or Eu²⁺-activated fluoride phosphorsheretofore considered to have large temperature quenching, andconsequently, this embodiment can be achieved. Therefore, in thewavelength conversion member, the ultraviolet light can be sufficientlysubjected to the wavelength conversion without lowering thetransmittance of the visible light or the infrared light, and it becomespossible to sufficiently enhance the output of the photovoltaic deviceeven at high temperature.

[Photovoltaic Device]

Next, a description will be made of the photovoltaic device according tothis embodiment. The photovoltaic device according to this embodimentincludes the above-mentioned wavelength conversion member. Specifically,as the photovoltaic device according to this embodiment, a solar cellmodule 1 as shown in FIG. 1 can be mentioned as the photovoltaic deviceaccording to this embodiment.

As shown in FIG. 1, the solar cell module 1 includes: a solar cell 10 asa photoelectric conversion element; a wavelength conversion member 20disposed on a light receiving surface 13 side of the solar cell 10; anda surface protection layer 30 disposed on a surface of the wavelengthconversion member 20. The solar cell module 1 further includes: a backsurface sealing member 40 disposed on a back surface 14 that is asurface opposite with the light receiving surface 13 of the solar cell10; and a back surface protection layer 50 disposed on a back surface ofthe back surface sealing member 40. That is, the solar cell module 1 hasa configuration in which the surface protection layer 30, the wavelengthconversion member 20, the solar cell 10, the back surface sealing member40 and the back surface protection layer 50 are provided in this orderfrom above in the drawing.

The solar cell 10 absorbs light made incident from the light receivingsurface 13 of the solar cell 10, and generates photovoltaic power. Thesolar cell 10 is formed, for example, by using a semiconductor materialsuch as crystalline silicon, gallium arsenide (GaAs), indium phosphide(InP), or the like. Specifically, the solar cell 10 is formed, forexample, by laminating the crystalline silicon and amorphous silicon.Electrodes (not shown) are provided on the light receiving surface 13 ofthe solar cell 10 and on the back surface 14 that is the surfaceopposite with the light receiving surface 13. The photovoltaic powergenerated in the solar cell 10 is supplied to the outside via theelectrodes.

The wavelength conversion member 20 is disposed on the light receivingsurface 13 of the solar cell 10. As shown in FIG. 1, the wavelengthconversion member 20 includes: a sealing material 21 that seals thelight receiving surface 13 of the solar cell 10; and a fluoride phosphor25 dispersed in the sealing material 21. The wavelength conversionmember 20 prevents moisture from entering the solar cell 10 by thesealing material 21, and enhances strength of the entire solar cellmodule 1.

The surface protection layer 30 is provided on the light receivingsurface 13 side of the solar cell 10, protects the solar cell 10 fromthe external environment, and in addition, transmits therethrough lightto be absorbed by the solar cell 10. For the surface protection layer30, for example, a glass substrate can be used. Note that, besides theglass substrate, the surface protection layer 30 may be polycarbonate,acrylic resin, polyester and polyethylene fluoride. The back surfaceprotection layer 50 is a back sheet provided on the back surface 14 sideof the solar cell 10. The back surface protection layer 50 may be thesame transparent substrate as the surface protection layer 30, such asglass and plastic.

The back surface sealing member 40 is disposed on the back surface 14 ofthe solar cell 10, prevents moisture from entering the solar cell 10,and enhances the strength of the entire solar cell module 1. The backsurface sealing member 40 is made, for example, of the same material assuch a material that can be used for the sealing material 21 of thewavelength conversion member 20. The material of the back surfacesealing member 40 may be the same as or different from the material ofthe sealing material 21 of the wavelength conversion member 20.

Moreover, metal foil or the like may be provided between the backsurface sealing member 40 and the back surface protection layer 50 sothat the light made incident from the surface protection layer 30 sidecan be absorbed more by the solar cell 10. In this way, the light thathas reached the back surface protection layer 50 from the surfaceprotection layer 30 can be reflected toward the solar cell 10.

When the solar cell module 1 is irradiated with sunlight includingultraviolet light 70, visible light and infrared light 80, theultraviolet light 70, the visible light and the infrared light 80 passthrough the surface protection layer 30, and is made incident onto thewavelength conversion member 20. The visible light and the infraredlight 80 made incident onto the wavelength conversion member 20 pass asthey are through the wavelength conversion member 20 without beingconverted substantially by the fluoride phosphor 25, and then areapplied to the solar cell 10. Meanwhile, the ultraviolet light 70 madeincident onto the wavelength conversion member 20 is converted into thevisible light and the infrared light 80, which are light on the longwavelength side, by the fluoride phosphor 25, and thereafter, areapplied to the solar cell 10. The solar cell 10 generates photovoltaicpower 90 by the applied visible light and infrared light 80, and thephotovoltaic power 90 is supplied to the outside of the solar cellmodule 1 via the terminals which are not shown.

As mentioned above, in the wavelength conversion member 20 of thisembodiment, the fluoride phosphor 25 is used, in which the decrease ofthe internal quantum efficiency at high temperature is greatlysuppressed, and excellent temperature characteristics are imparted.Therefore, the ultraviolet light can be effectively utilized withoutlowering the transmittance in the visible light or the infrared light,and it becomes possible to enhance the output of the solar cell module 1even at high temperature.

EXAMPLES

Hereinafter, this embodiment will be described more in detail byExamples and Comparative Examples; however, this embodiment is notlimited to these Examples.

Examples 1 to 16 and Comparative Example 1 (Preparation of Phosphor)

Fluoride phosphors of Examples 1 to 16 and Comparative Example 1 weresynthesized by using a preparation method that utilizes the solid phasereaction, and characteristics thereof were evaluated. Note that, inthese Examples and Comparative Example, the following compound powderswere used as raw materials.

Barium fluoride (BaF₂): purity 3N, made by Wako Pure ChemicalIndustries, Ltd.

Strontium fluoride (SrF₂): purity 2N5, made by Wako Pure ChemicalIndustries, Ltd.

Calcium fluoride (CaF₂): purity 3N, made by Kojundo Chemical LaboratoryCo., Ltd.

Magnesium fluoride (MgF₂): purity 2N, made by Wako Pure ChemicalIndustries, Ltd.

Europium fluoride (EuF₃): purity 3N, made by Wako Pure ChemicalIndustries, Ltd.

Regarding Examples 1 to 16, first, the respective raw materials wereweighed at ratios shown in Table 1. Next, the raw materials werethoroughly dry-blended by using a magnetic mortar and a magnetic pestle,and each of baking raw materials was obtained. Thereafter, the bakingraw material was transferred to an alumina crucible, and was baked for 2hours in a reducing atmosphere (in a mixed gas atmosphere of 96%nitrogen and 4% hydrogen) at a temperature of 850° C. by using a tubularatmosphere furnace. Thereafter, a baked product thus obtained wasdisintegrated by using the alumina mortar and the alumina pestle,whereby each of the phosphors of Examples 1 to 16 was obtained.

Note that, when each of the obtained phosphors of Examples 1 to 16 wasirradiated with ultraviolet rays (wavelength: 365 nm), blue fluorescencewas visually observed in each case.

Regarding Comparative Example 1, first, the respective raw materialswere weighed at a ratio shown in Table 1. Next, the raw materials werethoroughly dry-blended by using the magnetic mortar and the magneticpestle, and a baking raw material was obtained. Thereafter, the bakingraw material was transferred to the alumina crucible, and was baked for2 hours in the reducing atmosphere (in the mixed gas atmosphere of 96%nitrogen and 4% hydrogen) at a temperature of 1200° C. by using thetubular atmosphere furnace. Thereafter, a baked product thus obtainedwas disintegrated by using the alumina mortar and the alumina pestle,whereby the phosphor of Comparative Example 1 was obtained.

Note that, when the obtained phosphor of Comparative Example 1 wasirradiated with the ultraviolet rays (wavelength: 365 nm), bluish purplefluorescence was visually observed.

TABLE 1 BaF₂ SrF₂ CaF₂ MgF₂ EuF₃ Composition (g) (g) (g) (g) (g) Example1 (Ba_(2.191)Ca_(0.8)Eu_(0.009))Mg₄F₁₄ 1.921 0.000 0.312 1.246 0.009Example 2 (Ba_(2.17)Ca_(0.8)Eu_(0.03))Mg₄F₁₄ 1.902 0.000 0.312 1.2460.031 Example 3 (Ba_(2.11)Ca_(0.8)Eu_(0.09))Mg₄F₁₄ 1.850 0.000 0.3121.246 0.094 Example 4 (Ba_(2.05)Ca_(0.8)Eu_(0.15))Mg₄F₁₄ 1.797 0.0000.312 1.246 0.157 Example 5 (Ba_(1.99)Ca_(0.8)Eu_(0.21))Mg₄F₁₄ 1.7450.000 0.312 1.246 0.219 Example 6 (Ba_(1.9)Ca_(0.8)Eu_(0.3))Mg₄F₁₄ 1.6660.000 0.312 1.246 0.313 Example 7 Ba_(2.21)Ca_(0.7)Eu_(0.09)Mg₄F₁₄ 1.9370.000 0.273 1.246 0.094 Example 8 Ba_(2.01)Ca_(0.9)Eu_(0.09)Mg₄F₁₄ 1.7620.000 0.351 1.246 0.094 Example 9 Ba_(1.94)Ca_(0.97)Eu_(0.09)Mg₄F₁₄1.701 0.000 0.379 1.246 0.094 Example 10 Ba_(1.91)CaEu_(0.09)Mg₄F₁₄1.674 0.000 0.390 1.246 0.094 Example 11Ba_(2.01)Sr_(0.1)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 1.762 0.063 0.312 1.246 0.094Example 12 Ba_(1.81)Sr_(0.3)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 1.587 0.188 0.3121.246 0.094 Example 13 Ba_(1.61)Sr_(0.5)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 1.4110.314 0.312 1.246 0.094 Example 14Ba_(2.11)Ca_(0.7)Sr_(0.1)Eu_(0.09)Mg₄F₁₄ 1.850 0.063 0.273 1.246 0.094Example 15 Ba_(2.11)Ca_(0.5)Sr_(0.3)Eu_(0.09)Mg₄F₁₄ 1.850 0.188 0.1951.246 0.094 Example 16 Ba_(2.11)Ca_(0.3)Sr_(0.5)Eu_(0.09)Mg₄F₁₄ 1.8500.314 0.117 1.246 0.094 Comparative Ca_(0.997)Eu_(0.003)F₂ 0.000 0.0007.784 0.000 0.063 example 1

(Evaluation) <Internal Quantum Efficiency>

The internal quantum efficiency of each of the phosphors obtained inExamples 1 to 16 and Comparative Example 1 was measured. The quantumefficiency of each of the phosphors was measured by using the quantumefficiency measurement system QE-1100 manufactured by Otsuka ElectronicsCo., Ltd. Measurement and analysis conditions are as follows.

Excitation wavelength: 350 nm

Number of integrations: 30 times

Exposure time: automatic

Measurement temperature range: 30° C. to 200° C.

Measurement temperature step: 10° C.

Excitation light wavelength range: ±20 nm

Fluorescence wavelength range: 370 nm to 800 nm

FIG. 2 is a graph showing relative internal quantum efficiency at eachtemperature when the internal quantum efficiency at 30° C. is taken as100% for the phosphors of Example 3 and Comparative Example 1. As shownin FIG. 2, it is understood that the phosphor of Example 3 exhibits highinternal quantum efficiency with respect to the phosphor of ComparativeExample 1 even at high temperature. Specifically, the relative internalquantum efficiency of the phosphor of Comparative Example 1 at 80° C. is80% or less, whereas the relative internal quantum efficiency of thephosphor of Example 3 at 80° C. is 98%. Moreover, even when thetemperature of the phosphor of Example 3 rose, the phosphor of Example 3exhibited high relative internal quantum efficiency, which is 96% at100° C. and 89% at 150° C.

Table 2 shows relative internal quantum efficiency at 80° C. withrespect to the internal quantum efficiency at 30° C. ([internal quantumefficiency at 80° C.]/[internal quantum efficiency at 30° C.]×100) ineach of the phosphors of Examples 1 to 16 and Comparative Example 1.Moreover, Table 2 also shows an emission peak wavelength when each ofthe phosphors of Examples 1 to 16 and Comparative Example 1 was excitedat 350 nm.

As shown in Table 2, each of the phosphors of Examples 1 to 16 exhibitedlight emission in a visible light region of 400 nm or more. Moreover, ineach of the phosphors of Examples 1 to 16, the relative internal quantumefficiency at 80° C. with respect to the internal quantum efficiency at30° C. was 95% or more, and each of the phosphors exhibited superiortemperature characteristics to those of the phosphor of ComparativeExample 1.

TABLE 2 Emission peak Relative internal quantum efficiency wavelength at80° C. with respect to internal Composition (nm) quantum efficiency at30° C. (%) Example 1 (Ba_(2.191)Ca_(0.8)Eu_(0.009))Mg₄F₁₄ 420 99 Example2 (Ba_(2.17)Ca_(0.8)Eu_(0.03))Mg₄F₁₄ 448 97 Example 3(Ba_(2.11)Ca_(0.8)Eu_(0.09))Mg₄F₁₄ 458 98 Example 4(Ba_(2.05)Ca_(0.8)Eu_(0.15))Mg₄F₁₄ 460 98 Example 5(Ba_(1.99)Ca_(0.8)Eu_(0.21))Mg₄F₁₄ 460 98 Example 6(Ba_(1.9)Ca_(0.8)Eu_(0.3))Mg₄F₁₄ 457 98 Example 7Ba_(2.21)Ca_(0.7)Eu_(0.09)Mg₄F₁₄ 460 97 Example 8Ba_(2.01)Ca_(0.9)Eu_(0.09)Mg₄F₁₄ 456 98 Example 9Ba_(1.94)Ca_(0.97)Eu_(0.09)Mg₄F₁₄ 455 97 Example 10Ba_(1.91)CaEu_(0.09)Mg₄F₁₄ 456 97 Example 11Ba_(2.01)Sr_(0.1)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 450 97 Example 12Ba_(1.81)Sr_(0.3)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 441 97 Example 13Ba_(1.61)Sr_(0.5)Ca_(0.8)Eu_(0.09)Mg₄F₁₄ 436 98 Example 14Ba_(2.11)Ca_(0.7)Sr_(0.1)Eu_(0.09)Mg₄F₁₄ 455 96 Example 15Ba_(2.11)Ca_(0.5)Sr_(0.3)Eu_(0.09)Mg₄F₁₄ 450 98 Example 16Ba_(2.11)Ca_(0.3)Sr_(0.5)Eu_(0.09)Mg₄F₁₄ 444 99 ComparativeCa_(0.997)Eu_(0.003)F₂ 427 80 example 1

<Excitation Characteristics and Light Emission Characteristics>

Excitation and light emission characteristics of the phosphor of Example3 were evaluated. Specifically, an excitation spectrum and an emissionspectrum were measured by using the spectrofluorometer (FP-6500)manufactured by JASCO Corporation. Note that an excitation wavelength atthe time of measuring the emission spectrum was set to 350 nm, and amonitoring wavelength at the time of measuring the excitation spectrumwas set to an emission peak wavelength (458 nm).

As shown in FIG. 3, it is understood that the phosphor of Example 3absorbs the ultraviolet light having a wavelength of 300 nm or more toless than 400 nm, and exhibits light emission having a peak at 458 nm.

<Refractive Index>

Refractive indices of the phosphors of Example 3 and Comparative Example1 were measured. The refractive indices of the phosphors were measuredby the Becke line test (according to JIS K 7142 B method) by using theAbbe refractometer NAR-2T manufactured by Atago Co., Ltd. and thepolarizing microscope BH-2 manufactured by Olympus Corporation.Measurement conditions are as follows.

-   -   Immersion liquid: propylene carbonate (n_(D) ²³ 1.420)        -   butyl phthalate (n_(D) ²³ 1.491)    -   Temperature: 23° C.    -   Light source: Na (D line/589 nm)

As a result of the measurement, the refractive index of the phosphor ofExample 3 was 1.45, and the refractive index of the phosphor ofComparative Example 1 was 1.44. As described above, it is understoodthat the refractive index of the phosphor of Example 3 stays within arange of 1.41 or more to less than 1.57, and is close to the refractiveindex of the sealing material.

Example 17

A wavelength conversion member was produced by using the phosphor ofExample 3 and an ethylene-vinyl acetate copolymer (EVA) as the sealingmaterial. Specifically, the phosphor and the ethylene-vinyl acetatecopolymer were weighed at ratios shown in Table 3. As the ethylene-vinylacetate copolymer, Evaflex (registered trademark) EV450 manufactured byMitsui-Dupont Polychemicals Co., Ltd. was used.

Next, a mixture of the phosphor and the ethylene-vinyl acetate copolymerwas obtained by melt-kneading using the plastomill manufactured by ToyoSeiki Seisaku-sho, Ltd. at a heating temperature of 150° C. and a numberof revolutions of 30 rpm for 30 minutes. Then, the obtained mixture wassubjected to hot press by a hot press machine at a heating temperatureof 150° C. and a pressing pressure of 1.5 MPa, whereby a sheet-shapedwavelength conversion member having a thickness of 0.6 mm was obtained.

Comparative Example 2

A wavelength conversion member of this Example was obtained in the samemanner as in Example 17 except that a BAM phosphor (BaMgAl₁₀O₁₇: Eu²⁺)was used as the phosphor.

TABLE 3 Phosphor Sealing material Blending Blending quantity quantityTransmittance Type (g) Type (g) (%) Example 17 Example 3 11 EVA 46 81Comparative BAM 11 EVA 46 42 example 2

Transmittance of each of the wavelength conversion members obtained inExample 17 and Comparative Example 2 was measured. The transmittance wasmeasured by using the ultraviolet-visible-near infraredspectrophotometer UV-2600 manufactured by Shimadzu Corporation.Measurement conditions are as follows.

Measurement range: 300 to 800 nm

Scan speed: 600 nm/min

Sampling interval: 1 nm

Slit width: 2 nm

Light source switching wavelength: 340 nm

Light source (300-340 nm): deuterium lamp

Light source (340-800 nm): tungsten halogen lamp

Table 3 also shows the transmittance of light of 590 nm in thewavelength conversion members of Example 17 and Comparative Example 2.Note that the refractive index of the BAM phosphor is 1.77.

As shown in Table 3, the wavelength conversion member of Example 17,which used the phosphor of Example 3 exhibited a high transmittance of81%. Meanwhile, the wavelength conversion member of Comparative Example2, which used the BAM phosphor, had a low transmittance of 42%. This isbecause the refractive index of the phosphor of Example 3 is 1.45, whichis close to the refractive index (1.48) of the EVA, and the refractiveindex of the BAM phosphor is 1.77, which is greatly different from therefractive index of the EVA. That is, in the case of the BAM phosphor,since the difference in refractive index from the sealing material waslarge, the light that hit phosphor particles was scattered, and thetransmittance decreased. Meanwhile, in the case of the phosphor ofExample 3, the difference in refractive index from the sealing materialwas small, and light scattering was suppressed, and accordingly, hightransmittance was exhibited.

The entire contents of Japanese Patent Application No. 2015-022869(filed on: Feb. 9, 2015) is incorporated herein by reference.

Although the present invention has been described above by reference tothe embodiments and the Example, the present invention is not limited tothose, and it will be apparent to these skilled in the art that variousmodifications and improvements can be made.

INDUSTRIAL APPLICABILITY

The wavelength conversion member of the present invention uses thefluoride phosphor, in which the decrease of the internal quantumefficiency is suppressed, and excellent temperature characteristics areimparted. Therefore, it is possible to effectively utilize theultraviolet light even at high temperature, and it becomes possible toenhance the output of the photovoltaic device.

REFERENCE SIGNS LIST

-   -   1 SOLAR CELL MODULE (PHOTOVOLTAIC DEVICE)    -   20 WAVELENGTH CONVERSION MEMBER    -   21 SEALING MATERIAL    -   25 FLUORIDE PHOSPHOR

1.-6. (canceled)
 7. A wavelength conversion member comprising: afluoride phosphor activated by Ce³⁺ or Eu²⁺, the fluoride phosphorcomprising, as a host, a compound represented by a general formula:M₃Mg₄F₁₄, where M is alkaline earth metal, wherein, with regard to thefluoride phosphor, internal quantum efficiency measured at 80° C. is 85%or more when internal quantum efficiency measured at 30° C. is taken as100%.
 8. The wavelength conversion member according to claim 7, whereina refractive index of the fluoride phosphor is 1.41 or more to less than1.57.
 9. The wavelength conversion member according to claim 7, furthercomprising a sealing material which disperses the fluoride phosphor. 10.A photovoltaic device comprising the wavelength conversion memberaccording to claim 7.